Acrobat 10 doesn't accept serial numbers for international versions of Acrobat (or the CS suite) if the operating system is running in a different language than the purchased one. This hint discusses a few workarounds.

Acrobat comes with several language packages and runs by default with the language of the operating system. In this case however it doesn't accept serial numbers which are bound to a specific language. As it seems there are three work arounds (the first two are also mentioned by the Adobe support team):

The obvious one: Run OSX in the same language as Acrobat was purchased in. At least in my case not an option.

Use a program such as Language Switcher. I have not tried that but it probably does the same as option 3 below, only temporarily.

Remove the English language package (of course at your own risk): Open the Acrobat package, navigate to Contents/Resources and remove the folder 'en.lproj.'

[crarko adds: I haven't tested this one. I'd vote for option 2, I think. If you try option 3 be prepared to restore form backup/reinstall if you want to use English again.]

Instead of physically removing the English (or any other) lproj (and in the spirit of never hacking something when the OS provides a way to do said thing itself), can you not simply disable that localization in Acrobat using the Languages: section of the Finder's Get Info window for the app?

Setting a separate default language for the app via either Language Switcher (or the 'defaults' command it uses) is still probably a slightly better solution, since it makes absolutely no changes to the app itself, only to your preferences. (TBH, I'm a bit surprised Language Switcher works properly, given the way it writes the AppleLanguages key, but I guess the OS must be more tolerant of strange formats for this key than for others, since it does seem to work properly.)